I'm confused too.
Bibliolept's rewritten sentence is: "He slept like a contented log, if a log can be content."
So why didn't Tolkien wrote as "Sam slept through the night in deep content, if logs could/can be contented." ?????

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I also found:
"a corrupt text"; "spoke a corrupted version of the language"
Is there any specific difference between them? Can one use them interchangeably or does it depend on the adjective which is the preferable version (how about these two then)?

I'm not entirely sure, I would say they are usually interchangeable, but the subtle difference is the '...ed' ending implies 'doing', whereas the phrase without denotes a state.

Thus ' I am content', 'I was content', ' I am corrupt' are states of being.
' I am contented by eating', ' I was contented by/with their answers', 'I was corrupted by bad companions' denotes the state being caused.

'The program was corrupt' is interchangeable with, because it presupposes 'the program was corrupted (by a virus/person)'.

' He slept content (contented by something), if a log can be in a state of content (contented by anything)'

I'm not entirely sure, I would say they are usually interchangeable, but the subtle difference is the '...ed' ending implies 'doing', whereas the phrase without denotes a state.

Thus ' I am content', 'I was content', ' I am corrupt' are states of being.
' I am contented by eating', ' I was contented by/with their answers', 'I was corrupted by bad companions' denotes the state being caused.

'The program was corrupt' is interchangeable with, because it presupposes 'the program was corrupted (by a virus/person)'.

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Yes this is exactly how I understand it - although I think people nevertheless do use them interchangeably sometimes. Another example that perhaps is mixed up less is open/opened -